Xiao-Yan Zhao Beam Instrumentation Group Accelerator Center, IHEP BEPCII Background Issues: Beam Loss Measurement

BLM system Introduction Experiment Results Conclusion Outline

 BLM System Structure  Detector  Detector Location BLM System Introduction

The beam loss monitor (BLM) system is used for detecting or locating any possible excessive beam loss happened in the injection, or due to beam instabilities, or from bad vacuum, and so on. This system is a useful supplement to the beam diagnostic system for the troubleshooting.

The system structure of BEPCII BLM The system consists of detector, front-end electronics, CAN bus and PC. The pulse signals from detectors are fed to front-end electronics. The CAN bus and Ethernet are used to connect all front-end electronics to the PC and BLM data to the central Control Room, respectively.

The detector is sensitive to MIPs (minimum ionizing particles) produced when an accelerated particle hits the wall of the vacuum chamber. The detector consist of two PIN-diodes mounted face to face. An AND-gate detects the coincidence of pulses from the two PIN-diodes. MIPs cause ionizations in both PIN-diodes, a coincidence occurs and an output pulse is generated. Photons do not cause ionization in both PIN-diodes. So, the detector is not sensitive to photons. Maximum count rate: 10MHz Detector (Bergoz Instrumentation Co.)

 The detectors are installed in the inner side and outer side of the vacuum pipe near the downstream of the most of quadrupole magnets.  Total of 113 pairs, namely 226 detectors are installed in the BEPCII double-ring. Among them, 75 pairs locate in the outer ring and 38 pairs locate in the inner ring, respectively. Detector location

The BLM layout in the IR IP e+ e- 56#: 6m from IP 57#: 2.1m from IP 58#: 0.26m from IP 63#: 6m from IP 62#: 2.1m from IP 61#: 0.26m from IP

 Ring Collimator  Transport Line Collimator  Beam Abort  Beams Colliding Experiment Results

SR Injection, Current 0-257mASR Run, E=2.5GeV For Synchrotron Radiation mode, when the beam is injected or stored, the beam loss is small in the IR. So we only focus on the beam loss for the colliding mode.

Ring Collimator Experiment ( )

The beam loss and lifetime are measured by adjusting the collimator with different aperture during the beam stored.

Time I b （ mA,10bucket ） (e - beam) R4OCH02 (8m upstream the IP) Hori. Colli. position Outer(mm)Inner(mm) 14: : : : (14  ) 14: : : : : : : : : : : : (11.3  ) Change the collimator aperture, the beam lifetime reduces and the beam loss increases (58 ＃ :0.26m West from the IP ) 42mm(R4OCH02:inside and outside) 32m m 31m m 30m m 29m m 27m m 28m m 26m m After the aperture adjustment, the lifetime and beam loss recover quickly.

Similarly, the beam loss is measured by adjusting the collimator with different apertures during the beam injection.

During the e - beam injection, the beam loss is large. But the change of beam loss is not obvious, even if the collimator aperture is different. e-:0~89mA R4OCH02:42mm beam abort e-:107~155mA,R4OCH02:26mm e-:0~40mA R4OCH02:26mm e-:88~106mA R4OCH02:32mm Time I b （ mA ） (e - beam, Injection) R4OCH02 Collimator position Outer (mm)Inner (mm) 15:42:06- 15:42: :43:26-15:43: (14  ) 15:43:39-15:43: (11.3  ) 15:45:36-15:45: (11.3  ) 15:47:59-15:48: (11.3  ) e-:0~206mA R4OCH02:26mm

Transport line Collimator Experiment ( ) The beam loss is measured for several times when beam is injected. We find the injection of e - beam brings more beam losses, so we focus on the injection of e - beam especially.

In order to reduce the background during e - beam injection, The collimators located on the transport line are used for the control of beam energy spread and emittance by adjusting the collimators aperture.

Injection Run In order to reduce the injection background, we change the aperture of energy spread collimator and emittance collimator. Aperture 8  x Energy spread ±0.8% Aperture 3  x Energy spread ±0.3%

This picture shows the beam loss along the ring when e- beam is injected. Aperture 8  x Energy spread ±0.8% Aperture 3  x Energy spread ±0.3%

BLM near IP Energy spread ±0.8% aperture 8  x Energy spread ±0.3% aperture 3  x InsideOutsideInsideOutside 6m West from IP(56#)430,4402,250,15311,11371, m West from IP(57#)464,90639,23820,1443, m West from IP(58#)151,91762,1998,3305, m East from IP(61#)116,090112,9536,2357, m East from IP(62#)12,424142,3281,25711,225 6m East from IP(63#)69,77487,09412,52511,656 This table shows the beam loss comparison when we inject e- beam with different collimators aperture. The count of BLM in the IR reduces quickly after we set ±0.3% Energy spread and 3  x aperture.

Beam Abort Experiment The study of beam abort method is very important because there isn’t a special abort system in BEPCII.

beam abort The beam loss in the IR is large if we choose unfit methods when beam abort. So we optimize the method of beam abort by experiment the old method of beam abort the new method of beam abort

Beams Colliding In order to reduce the background of the second way, e - and e + beam are separated at the IP by local orbit bump. For the beam injection on the colliding mode, there are two ways to inject beam. One is to separate beam firstly during the injection by tuning the RF phase. Second way is to inject beam with colliding. But we found the second way bring more beam loss at the IP.

This picture shows the experiment result of injection with colliding e - and e + beam at the IP. e+:10mA, inject e-:0~10mA, Max beam loss≈5million/15s

This picture shows the experiment result which e - and e + beam are separated at the IP by local orbit bump. Inject e- e-:398mA, inject e+:0~400mA Max beam loss≈95,000/15s Abort e- e+:340mA, inject e-:0~330mA, Max beam loss≈160,000/15s

1.The injection of e - beam brings more beam losses than the injection of e + beam. 2.The ring collimator doesn’t show obvious effects to reduce the background. But the collimators on the transport line show clear effects to reduce the background during the electron beam injection. 3.The new beam abort method can obviously reduce the background. 4.If we use the injection way which separating the two beams by a local orbit bump, the background can be reduced. 5.For the SR mode, the beam loss in the IR is small. Conclusion

Thanks!